1. Field of the Invention
The present invention relates to a transparent thin film transistor and an image display unit. In particular the present invention relates to a transparent thin film transistor and an image display unit which can uniformly and steadily operate in which a wiring resistance is reduced.
2. Description of the Related Art
In recent years, the thin film transistor which used amorphous silicon, poly silicon, metal oxide semiconductors, organic semiconductors or the like as a driving transistor of electronic devices has been used. However, since amorphous silicon, poly silicon and organic semiconductors are optically sensitive in the region of visible light, light shielding layers are necessary.
On the other hand, transparent metal oxide semiconductors which have large band gaps are not optically sensitive in the region of visible light and can be deposited at low temperature. Therefore, a transparent thin film transistor which is flexible can be fabricated on a substrate such as a plastic substrate or a film (referred to as patent literature 1). The thin film transistor which used the metal oxide semiconductors attracts a lot of attention as the realization of the improvement in the aperture ratio of the active matrix display units or the realization of the new display structures.
In a general active matrix display unit, it is not necessary for a wiring to be transparent. Therefore, stacked films or alloy of metal materials such as aluminum (Al), copper (Cu), molybdenum (Mo) or tungsten (W) are mainly used for a general opaque thin film transistor. However, it is necessary not only for a semiconductor active layer but also for its wiring to be transparent to form a transparent thin film transistor. Now, indium tin oxide (ITO), indium zinc oxide, (IZO) or the like which is well-known as transparent conductive materials are used as a main material for the wiring.
However, when a wiring of an active matrix display unit is made of a transparent conductive material, an electrical resistivity thereof is high, compared with an electrical resistivity of metal materials. Therefore, it is difficult to drive a thin film transistor uniformly because of an effect of a wiring resistance when a panel size becomes large.
The present invention is to provide a transparent thin film transistor and an image display unit which can uniformly and steadily operate where a wiring resistance is reduced.    Patent Document 1: Japanese Patent Laid-Open No. 2000-150900 Official Gazette    Patent Document 2: Japanese Patent Laid-Open No. 2006-165528 Official Gazette    Non-Patent Document 1: K. Nomura et. Al., Nature, Vol. 432(2004-11), p. 488-492